Modern cellular communications systems employ satellite based links for relaying microwave signals between different Earth based stations, either or both of which may be mobile, and which may be located in different widely separated geographic regions. The satellite contains RF transponder systems that are capable of receiving and, through its microwave transmitter, relaying signals from many different stations on Earth to other stations simultaneously. A key component in that transponder system is the microwave transmitting (or receiving) antenna, which, typically, is a reflector antenna. A reflector antenna as is known employs a microwave feed horn and a parabolic reflector. Microwave energy emanating from the feed is directed onto the parabolic reflector and, thence, is radiated from that reflector into space.
Ideally, one would wish to communicate with all areas on Earth with a single satellite based cellular communication system. However, it is not technologically possible to realize that goal. The reality is that the geographic coverage of a single satellite system is much more limited in scope. The reason is principally two fold: the transmitted power level, that is the wattage, of the transponder's transmitter, and the directional characteristics of the transmitting antenna (or antennas).
The directional characteristic of the parabolic antenna is well known. Most of the RF energy fed to the antenna is radiated in a particular pattern, referred to as its principal lobe. The principal lobe is oriented in the desired direction along the reflector's parabolic axis, while some RF energy is radiated off axis, referred to as the side lobes. To visualize the shape of those lobes, and hence the antenna's directionality, using appropriate radiation measurement apparatus, one measures at the various angular positions about the antenna to find locations at which field strength or power, expressed in V.sub.1m (watts), bears a fixed ratio, suitably 6 dB, to that of the peak power, and those locations are plotted graphically relative to the angular distance from the antenna's axis. That technique provides a graphical outline or plot of that intensity. The shape of that plot is the antenna's directional characteristic.
The foregoing describes the antenna as a transmitting antenna. As those skilled in the art appreciate, the foregoing antennas are alternatively used both for transmitting and receiving microwaves using known transmitting and receiving apparatus. As further understood, the antenna is reciprocal in its electromagnetic characteristics. That is, it's directional characteristic for receiving is substantially the same characteristic obtained for transmitting microwave energy. Thus while this description speaks in terms of transmitting microwave energy for convenience and ease of description, it is expressly understood to apply also to the antenna when used in a receiving mode.
The principal lobe of a parabolic antenna is normally most intense along the antenna's axis and tapers off in any off-axis direction. The greater the angle off the axis, the lesser is the intensity, until in the radial direction, energy increases to form side lobes.
When those RF field measurements are taken along a plane perpendicular to the parabolic axis and plotted, a generally circular pattern is obtained for the principal lobe. Locations within the circle generally have greater intensity than points outside the circle. The latter situation is akin to the relationship of a parabolic antenna on board a satellite hundreds of miles or more above the earth, in which the antenna is directed toward a location on the earth.
From the reflector's position on the satellite radiating transmitted microwave energy to the Earth, and with the RF power directed into the reflector by the microwave feed being a constant, one finds a region on the earth where the level of received energy is sufficient for reliable telecommunications with the satellite. That region is called the antenna's "foot print". Outside of that region telecommunications are not reliable with normal communications receivers because the received RF signals are substantially at or below the receiver's electronic noise floor and become electronically unintelligible. Qualitatively, the foot print of the circular parabolic antenna is substantially a circle or, more accurately, a circle projected upon a sphere, which forms an ellipse. Should advances in communications receivers or higher power transmitters occur in the future, such more advanced equipment will of course enable one to expand the antenna's footprint to cover additional real estate on the Earth. Even with those improvements, however, those skilled in the art recognize that earth coverage of a single high gain antenna is not feasible.
In practice one finds that the antenna in the foregoing system possesses a foot print that does not cover a sufficiently large geographic region. To somewhat remedy that situation, multiple beam antenna systems have been proposed. Ideally, a multiple beam system would produce a series of separate beams of microwave radiation whose individual footprints on the Earth are substantially contiguous with one another and may have some slight overlap. To uniformly accomplish the foregoing reception pattern requires the formed beams to be highly circular in symmetry, the main beam or lobe possesses a steep "rolloff" and produces low sidelobes to avoid interference to surrounding areas covered by any other beams.
Each such beam originates from an associated microwave feed that is directed to a single reflector. A typical multiple beam antenna incorporates three or more distinct microwave feeds. Of necessity those feeds are constrained to a maximum size determined by the effective focal length and angular separation of adjacent beams. Often these are slightly overlapping to maintain high edge of coverage gain. With a constrained maximum feed size, the feed illumination of the parabolic reflector cannot have any desired amplitude distribution and the beam produced does not guarantee circular beam symmetry, steep main beam roll off and low side lobes.
As is known, the size of the microwave feed influences the spatial distribution of microwave energy reflected from the antenna's reflector. By size, reference is being made to the physical diameter of the outlet or exit of the microwave horn that serves to direct the microwave energy being transmitted onto the associated reflector from whence that energy is radiated into space. The smallest size feed produces a beam that more uniformly radiates the full surface of the reflector including the reflector's edges and beyond, producing a narrow principal lobe to the beam, but also, disadvantageously, producing high side lobes as well. Since the side lobes are directed off boresight, and not toward the angle at which the reflector's axis is directed, the energy in those side lobes is essentially lost, or wasted or interferes with adjacent coverage area beams. To better concentrate more of the radiation into the principal lobe, one normally thus employs a larger sized microwave feed.
With a larger sized microwave feed, the energy radiated by the feed toward the reflector is more focused, that is, is more confined to the reflector's central area and less or none to the reflector's outer edges. The effect is to maximize the principal lobe, and minimize the side lobes, thereby using the microwave energy emanating from the microwave feed more efficiently. The latter arrangement is also found to produce an additional effect that is beneficial to the present invention. The "roll-off" of the beam is enhanced. That is, the principal lobe's intensity drops off more quickly as the boresight angle off the reflector axis attains a particular angle and becomes negligible as the angle increases there beyond, until the vicinity of the low-level side lobes is attained at extreme off-axis locations. The latter is the accepted engineering practice for a single beam antenna.
A multi-beam antenna requires many individual microwave feeds that use a single parabolic reflector in common. At most, only one of those feeds can be located at the reflector's focal point. Attempting to take advantage of the benefit of the large size microwave feeds, one finds that placing a number of large size feeds side by side in a focal plane confronting the reflector takes up too much space. Apart from the one feed that may be located at the focal point, the remaining feeds are displaced too far from the focal point to provide the kind of spatial radiation of the reflector necessary to obtain the desired direction of radiation characteristics achieved in the single beam antenna. As a consequence, the microwave beams produced cover separate regions of the Earth that are disconnected from one another, that is, are discontinuous; their respective footprints are separated. Such an antenna structure is therefore unacceptable for cellular communications systems where continuity of real estate coverage is desired. The obvious physical constraint renders that impractical for the multi-beam configuration.
Of necessity therefore, existing multi-beam satellite cellular communications antennas continue to use small size microwave feeds, notwithstanding the described inefficiencies.
The multi-beam satellite cellular communications antenna of the present invention also employs small size microwave feeds. However, applicant has discovered the means to make those small size microwave feeds emulate the large size feeds. The invention thus accepts the physical limitation on feed size while obtaining the beneficial spatial characteristics of the larger sized feeds. That emulation is achieved through recognition of a previously unrecognized effect incident to resistive tapering of reflectors and application of that effect within a multi-beam antenna. An interesting phenomenon recognized in the prior art literature is that a resistive coating on the parabolic reflector can be used to reduce the antenna's side lobes, which is disclosed in U.S. Pat. No. 5,134,423, granted Jul. 28, 1992 to Haupt (the "Haupt" patent). Unrecognized in the Haupt patent and discovered by the present inventor, is that the resistive coating also has an effect on the characteristics of the antenna's principal lobe. In achieving the new multi-feed antenna, the present invention also makes use of a resistive taper on the parabolic reflector, capitalizing upon and quantifying that previously unrecognized effect.
Accordingly, an object of the present invention is to provide a new multi-beam satellite antenna structure.
An additional object is to provide a parabolic antenna with a small size microwave feed that emulates a prior parabolic antenna containing a large size microwave feed.
A still additional object of the invention is to produce a multi-beam microwave antenna whose beams provide coverage of contiguous regions on Earth.
And a further object of the invention is to provide in a satellite antenna structure multiple contiguously positioned small sized microwave feeds that electromagnetically emulate microwave feeds of a larger physical size.